Densities in formations (layers) are usually assumed to be constant for gravity model calculations. This constancy implies that formations are homogeneous and isotropic. However, formations are usually heterogeneous, and densities vary depending on heterogeneity. For this reason, density should be considered a variable, and certain scientists do just this in their model calculations for each formation. In other words, density is defined as a function of the required parameters. In fact, functional change is regular, whereas density is an irregular variable that depends on the changing boundaries of seismic velocity. This study aims to look at density as a variable by using determined seismic velocity boundaries at which the seismic velocity changes for each formation. The change boundaries of seismic velocity are an indication of the change of density in the formation. It is not possible for the respective description of this change to be functional for each point. In addition to defining the main formations in model geometries using 3D inversion calculations, this study defines another formation, which is described by a combination of all of the change boundaries of seismic velocity that are present in each formation in specific order. The reliability of the results of the method depend on the reliability of the seismic velocity boundaries. In addition, an increasing number of seismic velocity boundaries lead to a higher resolution of density variations.